AU2017216250B2 - Method and cell for conversion of dinitrogen into ammonia - Google Patents

Abstract

The invention relates to a method and an electrochemical cell comprising a cathodic working electrode comprising a nanostructured catalyst, a counter electrode and an electrolyte for the reduction of dinitrogen to ammonia. The invention includes introducing dinitrogen and a source of hydrogen to the electrolyte, wherein the dinitrogen is reduced to ammonia at the cathodic working electrode. The electrolyte comprises one or more liquid salts formed from the combination of a specified set of cations and a specified set of anions.

Description

METHOD AND CELL FOR CONVERSION OF DINITROGEN INTO AMMONIA

FIELD OF INVENTION

[0001] The present invention relates to an electrochemical apparatus and method for the conversion of dinitrogen (N₂) into ammonia.

[0002] In one form, the invention relates to the cathodic reduction of dinitrogen.

[0003] In one particular aspect the present invention is suitable for use in industrial production of ammonia.

BACKGROUND ART

[0004] It is to be appreciated that any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the present invention. Further, the discussion throughout this specification comes about due to the realisation of the inventor and/or the identification of certain related art problems by the inventor. Moreover, any discussion of material such as documents, devices, acts or knowledge in this specification is included to explain the context of the invention in terms of the inventor's knowledge and experience and, accordingly, any such discussion should not be taken as an admission that any of the material forms part of the prior art base or the common general knowledge in the relevant art in Australia, or elsewhere, on or before the priority date of the disclosure and claims herein.

[0005] Ammonia production is a highly energy intensive process, consuming 1 -3% of the world electrical energy and about 5% of the world natural gas production. World production is currently around 200 million tonnes annually, reflecting the vast need for this chemical in agriculture, pharmaceutical production and many other industrial processes.

[0006] Ammonia is also being considered as a carbon-free solar energy storage material, due to its useful characteristics as a chemical energy carrier. Compared to other chemicals that could be used to store solar energy (such as hydrogen), ammonia is safe, eco-friendly and, most importantly, produces no CO2 emissions. Once stored in this form, the energy is readily recovered via the ammonia fuel cell.

[0007] For more than a hundred years, ammonia has been produced from dinitrogen and hydrogen in the presence of an iron based catalyst at high pressures and high temperatures according to the following reaction:

iron— based catalyst

N₂₍„₎ + 3H?_(jf) ========± 2NH₃₍„₎

15-30 Pa;430-480°C * ,. ,

Equation 1

[0008] This process, known as the Haber-Bosch process has been of key importance in producing the inexpensive fertilisers that have supported the large global population growth over the past century. The Haber-Bosch process uses very high temperatures and pressures, and requires substantial amounts of energy in the form of natural gas, oil or coal for the production of the required hydrogen.

[0009] Given the need to feed a growing world population, whilst simultaneously reducing global carbon emissions, it is highly desirable to break the link between industrial nitrogen-based fertiliser production and the use of fossil fuels. Therefore, there is intense interest in alternative pathways for ammonia synthesis.

[0010] The ideal system for the conversion of dinitrogen into ammonia would be economically feasible and easily scalable, would operate at ambient condition and be coupled to renewable energy sources such as wind, hydro or solar. An electrochemical approach is potentially capable of achieving this aim. The electrochemical reduction process involves dinitrogen gas as a starting material and uses various aqueous electrolytes or H₂ gas as the source of the H⁺. The electrochemical reduction of dinitrogen largely depends on the structure, components, and surface morphology of the electrocatalyst. (van der Ham et al, Chemical Society Reviews 43, 5183-5191 (2014)).

[001 1] For example, Koleli & Ropke have investigated polyaniline electrodes in methanol/LiCI0₄/H⁺ solution and achieved a maximum current efficiency of 16% at - 0.12V (vs normal hydrogen electrode) at room temperature and elevated pressure. (Koleli, F. & Ropke, T. Applied Catalysis B-Environmental 62, 306-310, (2006))

[0012] By using a membrane electrode assembly based cell with Pt electrodes Lan et al have achieved an ammonia production rate of 1 .14x10^~5 mol.m^"2.s^"1 from air and water at ambient temperature and pressure and an overall cell voltage of 1 .6 V. (Lan et al, Scientific Reports 3, (2013)).

[0013] Ammonia has also been produced using a mixture of N2 and steam in a molten hydroxide suspension of nano-Fe20₃ at a cell voltage of 1 .2 V and columbic efficiency of 35%. (Licht et al. Science 345, 637-640 (2014)). Although this advance shows great promise for competition with current ammonia industry processes, the high temperature used (~200°C) still requires significant input of heat and energy. So far, electrochemical conversion has not been sufficiently successful to be considered as a viable replacement for the Haber-Bosch process. Furthermore, electrochemical conversion of the prior art has not reached sufficiently high efficiency levels such as those exhibited by dinitrogen- fixating bacteria. (Rosea et al, Chemical Reviews, 2009, 109, 2209-2244).

[0014] US patent application 2006/0049063 and US patent 6,712,950 teach the synthesis of ammonia gas by anodic reaction from nitrogen-containing species or dinitrogen gas, and hydrogen-containing species or hydrogen gas in a non-aqueous liquid electrolyte such as a molten salt or an ionic liquid. The method involves the production of the N₃ ^" ion in the electrolyte and then the reaction of the N₃ ^" ion at the anode to produce ammonia. This method is limited by the need for the medium to be selected such that it can dissolve useful amounts of the N₃ ^" to support practical rates of ammonia production.

[0015] U.S. patent application 2016/0138176 (Yoo et al) describes a method of synthesizing ammonia using an electrolysis cell containing an aqueous, or liquid electrolyte of an alkali metal (salt) or an ionic liquid. The disclosure includes teaching of the use of electrolytes formed from a wide range of cations and anions (many of which do not form liquid salts) and some of which are of limited stability and utility with regard to efficiency of ammonia synthesis. [0016] There is therefore an ongoing need to identify new electrolytes and improved methods for cathodic dinitrogen reduction leading to ammonia synthesis.

[0017] In contrast to hydrogen generation or CO2 reduction, very few prior art electro- catalysts or photo-catalysts have been reported to exhibit useful activity for N₂ reduction. Little is known about the requirements or possible mechanisms for such reactions. However it is known that to become commercially viable, electrochemical conversion of dinitrogen has to overcome the obstacles presented by the high stability and chemically inert nature of dinitrogen.

[0018] Thus far, catalyst development has focused on catalysts for thermal synthesis of ammonia. Current electrochemical catalysts for ammonia synthesis have been limited to conductive polymers (Koleli, F. & Ropke, T. Applied Catalysis B-Environmental 62, 306-310, (2006)), meso Ni-Cu alloy (Licht et al. Science 345, 637-640 (2014)), and commercial Pt/C catalysts (Lan et al, Scientific Reports 3, (2013)).

[0019] Metallic nano-catalysts have been widely used for fuel cells and C0₂ reduction. It is well-known that the performance of nanocatalysts depends on their particle size, morphology and crystal structure.

[0020] Greenlee et al report the synthesis of a NiFe nanocatalyst for electrochemical ammonia synthesis (Nov. 8-13, 2015, AIChE annual meeting, Salt Lake City, UT https://aiche.confex.com/aiche/2015/webprogram/Paper422270.html). Further characterisation or performance of the catalyst was not reported.

[0021] Prior art electrocatalysts typically do not have high enough efficiency, catalytic activity and stability for dinitrogen reduction. In addition, the low solubility of dinitrogen in water (20 mg/L, 20^°C, 1 bar) leads to low reaction rates in prior art reports.

SUMMARY OF INVENTION

[0022] An object of the present invention is to provide an efficient electrochemical process for production of ammonia. [0023] Another object of the present invention is to provide an electrochemical cell suitable for an electrochemical process for cathodic dinitrogen reduction.

[0024] Another object of the present invention is to provide an improved method for cathodic dinitrogen reduction.

[0025] A further object of the present invention is to alleviate at least one disadvantage associated with dinitrogen cathodic reduction processes of the prior art.

[0026] It is an object of the embodiments described herein to overcome or alleviate at least one of the above noted drawbacks of related art systems or to at least provide a useful alternative to related art systems.

[0027] In a first aspect of embodiments described herein there is provided a cell for electrochemical reduction of dinitrogen to ammonia, the cell comprising: a cathodic working electrode comprising a nanostructured catalyst for reduction of dinitrogen, a counter electrode, and an electrolyte comprising one or more liquid salts in contact with the working electrode wherein the liquid salt is formed by a combination of :

(i) a cation selected from the group consisting of PRi_-4 (phosphonium), NRi_-4 (tetra alkylammonium), C₄H8NR2 (pyrrolidinium) wherein each R group is independently linear, branched or cyclic and preferably comprises from 1 to 18 carbon atoms, optionally partially or completely halogenated, optionally including a heteroatom, optionally including a functional group preferably chosen from ethers, alcohols, carbonyls (acetates), thiols, sulphoxides, sulphonates, amines, azos or nitriles, and wherein two R groups may connect to form a monocyclic or heterocyclic ring; and (ii) an anion selected from the group consisting of (RO)_xPF6-_x (phosphate), (RO)xBF_4-x (borate), R'S0₂NS0₂R' (imide), R'S0₂C(S0₂R')(S0₂R') (methide), FS0₂NS0₂F, C₂0₄BF₂, C₂0₄PF₄, RC₂0₄BF₂, RC₂0₄PF₄, CF₃SO₃ (triflate), R'S0₃ (sulphonate), R'C0₂, (carboxylate), CF₃COO (trifluoroacetate), R'_XPF6_-X (FAP), R'_XBF_4-X wherein each R' group is independently linear, branched or cyclic and preferably comprises from 1 to 18 carbon atoms, optionally partially or completely fluorinated and optionally including a functional group, preferably chosen from ethers, alcohols, carbonyls (acetates), thiols, sulphoxides, sulphonates, amines, azos or nitriles and wherein two R' groups may connect to form a monocyclic or heterocyclic ring..

[0028] In a preferred embodiment the liquid salt is formed by a combination of a cation selected from the group consisting of C₄mpyr (butyl-methyl pyrrolidimium), Ρβ,ββ,η, (trihexyl tetradecyl phosphonium), P(C₂Rf)₄ (where R_f is a perfluoralkyi); and an anion selected from the group consisting of eFAP (C₂FsPF3), NfO (nonafluorobutane sulphonate), PFO (perfluorooctane sulphonate), FSI (bis(fluorosulphonyl)imide, NTf₂ (bis(trifluoromethylsulphonyl)imide), B(otfe)₄ (tetrakis(2,2,2-trifluoroethane)borate and CF3COO (trifluoroacetate).

[0029] In a particularly preferred embodiment, the liquid salt includes a cation selected from PRi_-4 (phosphonium) cations.

[0030] In a particularly preferred embodiment, the liquid salt includes an anion selected from RSO3 (sulphonate) cations, particularly perfluorobutanesulphonate or perfluoropropanesulphonate, or trifluorophosphates, particularly eFAP (tris(perfluoroethyl)trifluorophosphate).

[0031] In another preferred embodiment the cation and/or anion of the liquid salt is fluorinated or perfluorinated.

[0032] It is also particularly preferred that the liquid salt has a relatively high nitrogen solubility (compared, for example, to liquid salts of the prior art such as imidazolium salts and nitrile based anions such as dicyanamide) [0033] In a particularly preferred embodiment of the cell for electrochemical reduction of dinitrogen to ammonia, the reduction of dinitrogen to ammonia occurs principally in a region adjacent a three phase boundary on the working surface of the cathodic working electrode.

[0034] Typically the nanostructured electrocatalyst is applied to the working surface of the cathodic working electrode to create the gas/electrolyte/metal three phase boundary region where electrolysis principally takes place.

[0035] The electrolysis cell may include other features well known to those in the art for carrying out electrolytic reactions and controlling the current between the electrodes. For example, the electrolysis cell may be adapted to control the temperature or pressure of operation using well known means such as heaters, cooling units or pressurising means. In addition the cell may include an ultrasonic generator for generating sound waves of energy greater than 20 kHz.

[0036] The electrolysis cell preferably includes gas flow layers having the function of allowing introduction to the cell of a stream of gas comprising nitrogen and hydrogen or water vapour, and exit of gas containing ammonia. The ammonia is optionally collected external to the cell.

[0037] In a further embodiment an assembly may be formed when two or more cells according to the present invention are stacked in series. One or more of the stacked cells may additionally be folded or rolled. Gas can be introduced at any convenient location including from the "end" of the longest dimension of the assembly or from either side of the assembly.

[0038] In a second aspect of embodiments described herein there is provided a method for the electrochemical reduction of dinitrogen to ammonia, the method comprising the steps of:

(1 ) contacting a cathodic working electrode comprising a nanostructured catalyst with an electrolyte comprising one or more liquid salts, wherein the liquid salt is formed by a combination of : (i) a cation selected from the group consisting of PRi_-4 (phosphonium), NRi_₄ (tetra alkylammonium), C₄HsNR₂ (pyrrolidinium) wherein each R group is independently linear, branched or cyclic and preferably comprises from 1 to 18 carbon atoms, optionally partially or completely halogenated, optionally including a heteroatom, optionally including a functional group preferably chosen from ethers, alcohols, carbonyls (acetates), thiols, sulphoxides, sulphonates, amines, azos or nitriles, and wherein two R groups may connect to form a monocyclic or heterocyclic ring; and

(ii) an anion selected from the group consisting of (RO)_xPF6-_x (phosphate), (RO)_xBF_4-x (borate), R'SCbNSCbR' (imide), R'S02C(S02R')(S02R') (methide), FS0₂NS0₂F, C₂0₄BF₂, C₂0₄PF_4, RC₂0₄BF₂, RC₂0₄PF_4i CF3SO3 (triflate), R'S0₃ (sulphonate), R'C0₂, (carboxylate), CF₃COO (trifluoroacetate), R'_XPF6-_X (FAP), R'_XBF_4-X wherein each R' group is independently linear, branched or cyclic and preferably comprises from 1 to 18 carbon atoms, optionally partially or completely fluorinated and optionally including a functional group, preferably chosen from ethers, alcohols, carbonyls (acetates), thiols, sulphoxides, sulphonates, amines, azos or nitriles and wherein two R' groups may connect to form a monocyclic or heterocyclic ring.

(2) introducing dinitrogen and a source of hydrogen to the electrolyte, wherein the dinitrogen is reduced to ammonia at the cathodic working electrode.

[0039] The method may additionally include a step (3) comprising collecting ammonia generated at the cathodic working electrode, separating the ammonia from other liquids and gases present by using a separate trap or separation unit.

[0040] Typically the dinitrogen is reduced at the cathodic working electrode to ammonia in the presence of a source of hydrogen, preferably hydrogen gas or water. In a particularly preferred embodiment of the method, the dinitrogen gas is humidified with water vapour to a controlled degree and then the humidified gas is passed in a stream over the cathode where the dinitrogen is electrochemically reduced to form ammonia. [0041] Typically, when the source of hydrogen is water, the anodic counter electrode converts the hydroxyl ions formed at the cathode into water and oxygen.

[0042] The counter electrode may be placed in the same electrolyte as the working electrode or alternatively, it may be separated by some means such as an electrolyte membrane or separator material. In another embodiment the counter electrode may be located in a compartment which optionally contains a different electrolyte medium, such as an aqueous solution.

[0043] The counter electrode reaction may be water oxidation or another advantageous oxidation reaction such as sulphite oxidation.

Electrolyte

[0044] The electrolyte in contact with the working electrode may comprise one or more salts, and those salts may be in the solid or liquid states, or combinations thereof.

[0045] The electrolyte is typically in the form of a layer. Preferably the electrolyte includes a spacer or electrolyte membrane (which itself may act as an electrolyte), for example a polymer electrolyte such as Nafion™ or a Nafion™-liquid salt blend, or a gelled liquid salt electrolyte, or is an electrolyte soaked into a porous separator such as paper or Celeguard™.

[0046] In a further embodiment of the present invention, there is provided an electrolyte membrane comprising a thin layer of material combined with one or more liquid salts as herein described for use in the cell of the present invention.

[0047] Preferably the electrolyte of the present invention has high solubility for dinitrogen and low solubility for water. Preferably the dinitrogen solubility is at least 100 mg/L at the operating temperature and pressure of the cell and more preferably more than 200 mg/L; most preferably the solubility is greater than 400 mg/L. T he solubility of water in the liquid salt is preferably less than 5 weight% at the operating temperature and pressure of the cell, more preferably less than 2% and most preferably less than 1 %. Liquid Salt

[0048] Where used herein the term liquid salt is intended to refer to an electrolyte medium that is liquid at the temperature of use and that contains one or more salts (each of which may be solid or liquid in their pure states). The salts may be chosen from any suitable metal salts, organic salts, protic salts, complex ion salts or the like.

[0049] The liquid salt medium can also be formed by mixing solid salts to create a liquid salt of the desired characteristics.

[0050] The liquid salt medium may contain additional components including water or other molecular liquids that are miscible with the liquid salt electrolyte. As will be appreciated by those skilled in the art, such materials include dimethylsulfoxide, tetraglyme and other oligio- and poly-ethers, glutaronitrile and other high boiling point nitriles, trifluorotoluene and other wholly or partially fluorinated solvents and propylene carbonate and other carbonate solvents. Where the molecular liquid component is volatile, the N₂ gas stream can be pre-saturated by bubbling through a container of the molecular solvent prior to the gas stream passing into the reduction cell; any molecular liquid passing out of the cell in the product gas stream can be condensed or otherwise captured for re-use. Preferably the molecular liquid is fluorinated or perfluorinated. Preferably the molecular liquid is present in the liquid salt medium at a level between 90 vol% and 0.1 vol%, more preferably between 20 vol% and 0.2 vol% and most preferably between 50 vol% and 0.5 vol%.

[0051] The electrolyte comprising liquid salt provides an ion conductive, low water content medium in which the process reactions occur. The electrolyte of the present invention also offers the advantage of low, or zero volatility and some gases are more soluble in these electrolytes comprising liquid salts than in water. In particular, some electrolytes comprising liquid salts can provide an elevated solubility for N₂ gas (compared to aqueous and other electrolytes) thereby increasing the concentration of N₂ at the electrolyte/electrode interface.

[0052] In another aspect of embodiments described herein, the present invention provides an electrolyte medium for electrochemical reduction of dinitrogen to ammonia according to the method of the present invention, the medium comprising one or more liquid salts having high solubility for dinitrogen and low solubility for water.

[0053] In a preferred embodiment the electrolyte comprises one or more eFAP liquid salts because they exhibit high N₂ solubility (ref S. Stevanovic et al, Chem. Thermodynamics 59 (2013) 65-71 ).

[0054] The electrolyte comprising liquid salt can also exhibit a degree of hydrophobicity that offers a controlled water activity to the electrochemical reaction, that is high enough to support the N2 reduction, but not so high as to support rapid reduction of water to H₂. In a preferred embodiment the electrolyte comprises one or more hydrophobic liquids based on the Ρβ,ε,ε, Μ cation. In a particularly preferred embodiment the electrolyte is substantially comprised of the liquid salt, [P6,6,6,i4][ eFAP].

[0055] In a particularly preferred embodiment the electrolyte is preconditioned prior to use, such as, by contacting it with an aqueous hydroxide solution. Without wishing to be bound by theory, the preconditioning may introduce a trace amount of OH^" into the liquid salt that provides a defined proton activity in the electrolyte of the present invention.

[0056] The temperature range applied to the electrolyte materials in a cell according to the present invention may be between -35 and 200°C, preferably 0°C to 150 °C, and most preferably 15°C to 130°C.

[0057] The pressure range of application is typically between 0.7 bar nitrogen pressure to 100 bar nitrogen pressure, preferably 1 bar to 30 bar and most preferably 1 bar to 12 bar. The pressure may be continuous, that is constant, or constantly increasing over time. Alternatively the pressure may be discontinuous, that is pulsed over time - alternating between higher and lower pressure values within the aforesaid ranges.

[0058] A pressure-temperature combination can be chosen to allow elevated N₂ solubility and electrochemical kinetics and also to allow ammonia to be generated at close to, but below, its condensation point (for examplel O bar at 25°C or 40 bar at 79°C). [0059] Typically a continuous current will pass between the cathodic working electrode and the counter electrode, however in some applications such as a wind power photovoltaic panel power driven process an intermittent or pulsed current may be suitable.

Nanostructured Catalyst

[0060] In a third aspect of embodiments described herein there is provided a catalyst for electrochemical reduction of dinitrogen to ammonia, the catalyst comprising nanostructured materials having a high electrochemical working surface area, as indicated by a double layer capacity, measured in an adjacent electrolyte layer of greater than 0.1 mF/cm² and preferably greater 1 mF/cm²

[0061] Preferably the nanostructured catalyst comprises one or more metals in the form of elemental metal or inorganic compounds comprising metals. The nanostructured catalyst may be in the form of discrete particles or sheet or film or three dimensional structure. The nanostructured catalyst embodies morphological features that may be of any shape with at least one dimension in the range of 1 nm to 1000 nm.

[0062] Suitable metals include any of the transition metals or lanthanide metals including Fe, Ru, Mo, Cu, Pd, Ti, Ce and La as well as their alloys with other metals and semimetals.

[0063] The aforementioned metals may be surface decorated with an oxide or a sulphide of the metal, or a composite may be formed of the metal with its oxides or sulphides.

[0064] The catalysts may also comprise a metal complex consisting of two metals bridged by sulphides. Preferably the metals are Fe and Mo.

[0065] The catalyst nanoparticle film may preferably be prepared by a cyclic voltammetry or a pulsed voltammetry electrodeposition method. [0066] In another preferred embodiment the catalyst may comprise conductive polymer materials such as PEDOT.

[0067] In yet another preferred embodiment the catalyst may comprise doped carbon materials, particularly carbons doped with N and/or S.

[0068] The catalyst is preferably supported or decorated on an electrically conductive, chemically inert support. Suitable supports include fluorine-doped tin oxide, graphene, reduced graphene oxide, porous carbons, carbon cloth, carbon nanotubes, conducting polymers and porous metals.

[0069] Faradaic efficiency is a particular deficiency of related processes of the prior art. Faradaic efficiency can be used to describe the fraction of electric current that is utilised in the N₂ reduction reaction. The remaining fraction that is, (100 - Faradaic efficiency)⁰/), is consumed in undesirable side reactions including the production of H₂ and hydrazine. These bi-products represent wasted energy and may also require complex separation methods from the desired product. It is one of the purposes of the present invention to provide a method of relatively high Faradaic efficiency preparation of ammonia.

[0070] Other aspects and preferred forms are disclosed in the specification and/or defined in the appended claims, forming a part of the description of the invention.

[0071] In essence, embodiments of the present invention stem from the realization that the efficiency of ammonia production can be improved by choice of specific nonaqueous electrolytes that increase the concentration of dissolved N₂ gas and play a certain homogeneous catalysis role to increase the activity of dinitrogen in the reduction reaction while lowering the rate of undesirable competing reactions such as H₂ production. The efficiency is further improved by using specific nanostructured catalysts.

[0072] Advantages provided by the present invention compared with the processes of the prior art comprise the following: conversion of dinitrogen to ammonia with high Faradaic efficiency; • the reaction can be carried out at ambient temperature and pressure;

• greater solubility of dinitrogen in the electrolyte;

• increased activity of dinitrogen in the reduction reaction;

• lower rate of undesirable competing reaction such as H2 production;

• water can be been used as hydrogen source so that no extra hydrogen is needed;

• suitable catalysts can be utilised from cheap and earth abundant materials;

• no highly corrosive electrolytes.

[0073] Further scope of applicability of embodiments of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the disclosure herein will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0074] Further disclosure, objects, advantages and aspects of preferred and other embodiments of the present application may be better understood by those skilled in the relevant art by reference to the following description of embodiments taken in conjunction with the accompanying drawings, which are given by way of illustration only, and thus are not limitative of the disclosure herein, and in which:

FIG. 1 is a schematic diagram showing a typical electrochemical cell for N₂ reduction according to the present invention;

FIG. 2 is a schematic depiction of an N₂ reduction cell according to the present invention and based on hybrid electrodes. FIG.3 is a schematic depiction of a stacked N2 reduction cell arrangement according to the present invention showing cells as described in Figure 2 stacked in series connection.

ABBREVIATIONS

[0075] Where used herein the abbreviations refer to the following chemical species: B(otfe)₄ - tetrakis(2,2,2-trifluoroethanoxy)borate (B(OCH₂CF₃)₄) C₄mpyr - butyl-methyl pyrrolidinium EMIM - ethyl methyl imidazolium eFAP - tris(perfluoroethyl) trifluorophosphate FSI - bis(fluorosulphonyl)imide ((FSC^N) HMIM - hexyl methyl imidazolium NfO - nonafluoro butane sulphonate (CF3(CF2)3SC>3) NTf2 - bis(trifluoromethyl sulphonyl)amide Ρβ,6,6,ΐ4 - trihexyl tetradecyl phosphonium Pi,4,4,4— tri butylmethyl phosphnium PEDOT -poly(3,4-ethylene dioxythiophene) perfluorobutyl sulphonate (FgC₄S0₃) PFO - perfluoro octane sulphonate (Fi₇Cs SO3) PFB - perfluorobutyrate

R_f - perfluoro alkyl -(CF₂)_n-i(CF₃) where n = is an even number DETAILED DESCRIPTION

[0076] The present invention will be further described with reference to the following non-limiting examples. These examples explore the electrocatalytic activities of the new catalytic materials in liquid salt electrolytes.

[0077] The examples also illustrate the fabrication of electrodes based on various metal nanostructures (nanoparticles or films) using chemical and electrodeposition methods and investigation of their electrocatalytic properties in electrolytes comprising one or more liquid salts. By using electrolytes comprising liquid salts, dinitrogen was successfully converted to ammonia with Faradaic efficiency of >60% at ambient temperature and pressure, which exceeds room temperature efficiencies of processes of the prior art.

[0078] The method of the present invention utilising a liquid salt based process not only can significantly increase the solubility of dinitrogen but also play a certain homogeneous catalysis role to increase the activity of dinitrogen in the reduction reaction and at the same time lower the rate of undesirable competing reactions such as H₂ production.

[0079] N₂ electroreduction catalyst films based on Fe were prepared by electrodeposition of the Fe onto a commercially available fluorine-doped tin oxide or porous metal (or carbon) substrates. Fluorine-doped tin oxide coated glass is electrically conductive and ideal for use in a wide range of devices. Fluorine doped tin oxide is relatively stable under atmospheric conditions, chemically inert, mechanically hard, high- temperature resistant, has a high tolerance to physical abrasion. Stainless steel mesh or cloth is useful as a high surface area substrate.

[0080] The deposition electrolyte comprised 10 mM FeS0₄, 10 mM NaOH and 10 mM citric acid. The electrodeposition was controlled by a potentiostat using a three-electrode system using fluorine doped tin oxide glass, Ti mesh, and a saturated calomel electrode (SCE) as the working, counter and reference electrode, respectively. A typical electrodeposition of this type is conducted using a cyclic voltammetry method between - 1.8 V and -0.8 V. A black and shining film was formed after several cycles. Changing the number of cycles allows control of the thickness of film, and a typical film was electrodeposited by 10 cycles at a scan rate of 20 mV s^"1.

[0081] The film was characterised by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), and electrochemical surface area (ECSA) measurement.

N₂ cathodic Reduction

[0082] The electrochemical N₂ reduction was conducted in a three-electrode system by using the Fe/fluorine doped tin oxide (or porous metla) electrode and platinum wire as the reference and counter electrodes, respectively. Cyclic voltammetry was measured in a one-compartment cell, while the N₂ reduction was conducted in a separated cell by isolating the counter electrode in compartment with a glass frit. Typical reduction was carried out for 3 hrs at -1.2 V vs Pt.

[0083] The N₂ gas was bubbled through a water trap to saturate it before bubbling into the electrolyte. The synthesized ammonia carried in the exit gas was trapped by passing the gas stream through a weak acid solution (1 mM H₂S0₄).

Product analysis

[0084] The ammonia produced in the gas phase was bubbled and collected through 1 mM H₂S0₄ solution. The ammonia remaining in the ionic liquid after completion of the electroreduction was extracted by washing the ionic liquid using 1 mM KOH. The total amount of ammonia is the sum of the ammonia in the acid trap and the basic washing solution and was later quantified using the indophenol blue method.

[0085] In a typical analysis, 0.5 ml of 0.5 M sodium hypochlorite was added into 0.5 ml aliquot of solution, then it was added 0.05 ml of 1 M NaOH with 5% (by weight) of salicylic acid and 5% (by weight) of sodium citrate, and 0.01 ml of 0.5% (by weight) sodium nitroferricyanide. After 2 hrs, the absorption of the solutions at 700 nm was measured on a UV-Vis spectrometer. Control experiments were conducted to validate the analysis method by adding different concentrations of added NH₄CI to the 1mM H₂S0₄ or 1 mM KOH solutions.

[0086] The Faradic efficiency was calculated based on 6-electron process, that is; N₂ + 6e^" + 6 H₂0 <=> 2 NH₃ + 6 OH^"

[0087] FIG. 1 is a schematic diagram showing a typical electrochemical cell for N₂ reduction according to the present invention comprising a power source (1 ), cathode (2), membrane (3) and anode (4). The counter electrode reaction in the process may be water or hydroxide oxidation as illustrated. Alternatively where the desired product is the fertiliser ammonium sulphate, the counter electrode reaction may be S0₃ ^"2 to S0₄ ^"2. An advantage of the latter is that the total energy cost of the process is lower than when oxygen is the cathode reaction product.

Cell

[0088] An N₂ reduction cell according to the present invention and based on hybrid electrodes is described in the following paragraphs. This electrolysis cell is designed to optimally support electrolysis when the reactants which are one or more gases are only of low solubility in the electrolyte and therefore it is desirable to have the reaction occur principally in the region at or near the three phase boundary (that is, the boundary between the electrode material, the gas phase and the electrolyte) within the body of the hybrid electrode.

[0089] The hybrid electrode of this invention preferably comprises a porous conductive electrode material, optionally decorated with nanostructured catalyst, such that the electrode has a high electrochemically active surface area and has a multiplicity of pores allowing both gas access and electrolyte access within the body of the electrode.

[0090] In the present invention the gas remains adjacent one surface and the pores of the hybrid electrode are at least partially filled with the ion conducting material so that transfer of reactants and products occurs in the region where the reduction of N2 to NH3 occurs.

[0091] This avoids one of the severe limitations of prior art electrodes that are designed to support gas diffusion within the porous electrode; this diffusion process creates undesirable overpotentials and in-efficiency in the cell in operation.

[0092] In this embodiment the cathodic working electrodes are hybrid ion-conducting electrodes or "electrode membranes" prepared by creating porous metal membranes and impregnating the porous structure with an ion conducting material for example Nafion™ or a Nafion™ gel or a gelled liquid salt. Decorating the cathode with electrocatalysts creates a gas/electrolyte/metal three phase boundary region where electrolysis can take place.

[0093] The reactant gas is introduced directly to the electrocatalyst from the flowing gas. The reactant or product ions diffuse through the electrolyte part of the electrode membrane and the gaseous products pass directly into the flowing gas phase.

[0094] As shown in FIG 2 the cell according to this embodiment is comprised of two porous electrodes (1 1 ,12) separated by an electrolyte layer (13). Electrode 1 is the cathodic working electrode at which nitrogen is reduced and electrode (12) is the counter electrode (anode) at which water oxidation or other desirable oxidation takes place. On the rear side of each electrode is a spacer layer (14, 15) which conducts gases in and out of the cell.

[0095] The electrodes comprise layers of flexible and porous conductive electrode sheet materials that have the electro-catalyst (16) loaded onto the sheet and exposed directly to the gas layer (14,15) respectively. Optionally the catalysts (16) can be deposited throughout the structure of the electrode (11 ) to form a multiplicity of regions where gas, electrolyte, catalyst and conductive electrode come into contact. The porosity of the conductor sheet is preferably such that the sheet is porous from one face to the other. [0096] The porous conductive electrode material may be a macro- or micro- or nano- porous metal such as stainless steel, iron, nickel, copper, ruthenium or their alloys in the form of a mesh, foam or wool. Porous carbon materials including carbon cloth, carbon particle/binder composites, graphene and reduced graphene oxide and conducting polymers are also suitable as porous conductive electrode materials.

[0097] The external encasement encloses the cell. In one form of the cell the cell is rolled or z-folded.

[0098] In a further embodiment of the present invention an assembly is formed by a number of cells stacked in series. In this format the electrolyte in the cell layers (18) must be isolated from one another and the encasement (17) is a conductive material so that a series connection is created between cells. The stacked cells may also be folded or rolled.

[0099] The electrolyte can comprise a spacer or be in the form of an electrolyte membrane, for example a polymer electrolyte, including Nafion™ or gelled Nation™ or gelled liquid salt electrolyte, or an electrolyte soaked into a porous separator such as paper or Celeguard™ type materials that are well known in the battery field.

[0100] In one form of the assembly, the layers (1 1 , 13 and 12) are pressed together and then the electrolyte is gelled in-situ to produce a self adhering assembly.

[0101] Gas flow layers (14) have the function of allowing the introduction of a stream of flowing gas that contains nitrogen and water vapour. The exiting gas contains ammonia which is optionally trapped external to the cell.

[0102] The gas flow layer (15) is designed to allow entry or exit of the gaseous reactions or products produced on that side of the cell, optionally a flow of air or other gas can be introduced to dilute and carry this gas out of the cell. The gas flow layers (14,15) are created either by use of a highly porous separator or mesh, or by printing or otherwise creating a pattern on one or both sides of layer (17).

[0103] In the assembly of stacked cells the gas can be introduced at any convenient location including from the "end" of the longest dimension of the assembly or from either side of the assembly. Each of these arrangements requires alternate sealing arrangements of layers (17).

[0104] The whole assembly may be as little as 1 mm thick, maximizing the surface area and minimising the amount of electrolyte material needed.

Example 1. Preparation of nanostructured Fe catalyst

[0105] The following Example describes preparation of a nanostructured catalyst suitable for use in the present invention. A fluorine doped tin oxide electrode (5 x 5 mm) was placed in an electrochemical cell and connected as the cathode. A standard calomel electrode (SCE) acted as reference electrode and a Ti mesh served as the counter electrode. The electrolyte solution for production of the catalyst comprised 10 mM FeS0₄, 10mM NaOH, and 10 mM citric acid in water.

[0106] Electrodeposition of Fe was carried out by cycling the potential of the fluorine doped tin oxide electrode between - 0.8 and -1.8 V vs SCE at 20 mV/s for 10 cycles. The electrodeposited Fe nanostructured layer appeared as a porous black layer comprising nanoparticles on the fluorine doped tin oxide. The deposited material is composed of mainly metallic Fe with homogenous distribution of partially oxidized iron (with 10% oxides). The iron oxide forms during the anodic part of the deposition cycle. The formation of iron oxide can control the iron nanoparticle size. The iron oxide is at least partially reduced during the initial stages of use as a cathode in ammonia production. The Fe/FeOxide composite may enhance the catalytic activity through a synergistic effect of the two oxidation states of iron.

[0107] The electrochemical surface area was determined by measuring the double layer charging current at 0 V vs Pt by cyclic voltammetry at 5mV/s in butyl methylpyrrolidinium eFAP liquid salt at room temperature. A double layer capacity of 2 mF/cm² was measured under these conditions. Example 2. Reduction of N2 in [P6,6,6,i4][eFAP]

[0108] The fluorine doped tin oxide electrode bearing the Fe electrocatalyst layer was placed in an electrochemical cell as the cathode. The electrolyte comprised the liquid salt [P6,6,6,i4 ][eFAP]. The electrolyte was preconditioned prior to use by shaking with 1 mM aqueous KOH solution. The aqueous solution was only sparingly soluble in the liquid salt, such that the excess solution can be removed as a separate layer in a separating funnel.

[0109] The preconditioning may introduce a trace amount of OH^" into the salts that provide a defined proton activity in the electrolyte.

[0110] The counter electrode (Pt) was placed in a fritted compartment, which also contains the saturated liquid salt. The reference electrode is Pt. Dinitrogen gas at 10 ml/min was introduced into the cell using a bubbler directing N₂ bubbles over the cathode. The dinitrogen was pre-saturated with water by bubbling through a solution of low water vapour pressure.

[011 1] The exiting gas flowing from the cell and containing unreacted dinitrogen and reduction products was directed into a further trap containing 1 mM H₂S0₄ to trap the ammonia produced as NH₄ ⁺. This solution was analysed for ammonia at the end of the electrolysis.

[0112] Electrolysis was carried out for 2 to 5 hours at voltages between -1.0 to -2.0 V. At the end of the electrolysis the electrolyte was thrice washed with 1 mM KOH solution to extract any retained ammonia. The three washes were also analysed for ammonia. The result was combined with the result form the trap solution analysis. The Faradaic efficiency was 73% (+/-5%).

[0113] The solutions were also analysed for hydrazine but no hydrazine was detected (at LOD = 5 μη οΙ/L). Example 3: Reduction of N2 in [C4mpyr][eFAP]

[0114] N₂ reduction was carried out as described in Example 2 except that the electrolyte comprised the liquid salt [C₄mpyr][eFAP]. The Faradaic efficiency in this case was 47% (+/-5%).

Example 4: Reduction of N2 in [P6,6,6,14 .[F^SOs]

[0115] Following the same methods as Example 2, the electrolyte comprised the liquid salt [P6,6,6,i4][FgC₄S0₃]. A constant current of 4 uA cm^"2 was applied for 2 hrs. Ammonia measurement showed that the Faradaic efficiency for ammonia was 51 %.

Example 5: Reduction of N2 in [Ρε,β,β,Η ][PFO]

[0116] Following the same methods as Example 2, the electrolyte comprised the liquid salt [Ρ_δ,θ,β_,π] [PFO]. A constant current of 4 uA cm^"2 was applied for 2hrs. Ammonia measurement shows that the Faradic efficiency% for ammonia was 39%.

Example 6: Reduction of N₂ reduction with assistance of sonication.

[0117] The following Example illustrates increased ammonia production by incorporating additional physical and/ or-chemical processes.

[0118] Following the same procedure as Example 2, the whole electrochemical cell was immersed into the bath of a sonicator. A constant potential of -1.2V vs NHE was applied on the working electrode for 30min, and sonication was applied to the electrochemical cell to promote the N₂ reduction. The current increased by approximately 125% compared to its previous value in the presence of the sonication.

Example 7: Reduction of N₂ in the presence of a molecular liquid component.

[0119] As mentioned previously the electrolyte can by a combination of two or more salts to create a liquid salt of the desired characteristics. Furthermore, the liquid salt may contain additional components including water or other molecular liquids as illustrated by the following Examples.

[0120] Following the same methods as Example 3, trifluorotoluene 10% v/v was added into the electrolyte as a further component to decrease the viscosity of the liquid salt and promote the mass transport in the electrochemical reaction. A constant potential of -1.2V vs NHE was applied for 30min. The current increases by 50% compared to its value in the absence of the trifluorotoluene.

Example 8: [C₄mpyr][perfluorobutanesulfonate]

[0121] [C₄mpyr][perfluorobutanesulfonate] has an elevated melting point (112°C) and could be used for N₂ reduction at temperatures above this point. The melting point can also be lowered by the addition of a molecular liquid component as described in Example 7.

Example 9: [C₄mpyr][PFO]

[0122] [C₄mpyr][PFO] has an elevated melting point (87°C) and could be used for N₂ reduction at temperatures above this point. The melting point can also be lowered by the addition of a molecular liquid component as described in Example 7.

Example 10: Preparation and Use of an electrolyte comprising a Gel Membrane containing a liquid salt

[0123] As described previously, the electrolyte used in the present invention may comprise a spacer or electrolyte membrane which itself is an electrolyte, for example a polymer electrolyte, such as Nafion™ or gelled liquid salt electrolyte, or is an electrolyte soaked into a porous separator such as paper or Celeguard™. The procedure set out below in Example 10, while illustrative of preparation of a PVDF-HFP copolymer gel electrolyte membrane, can be used more generally for preparation of any gel membrane containing a liquid salt.

[0124] A PVDF-HFP copolymer gel electrolyte membrane was prepared as follows using a liquid salt such as described in Example 3. A 9 to 1 ratio by weight of the liquid salt to copolymer was dissolved in dimethylformamide and heated slightly. The solution was spread onto a 34 mm diameter circle of Solupour™ membrane and left to dry. After drying, the excess gel on the outside of the membrane was removed.

[0125] A gas flow cell was constructed with a carbon paper/platinised carbon anode, the membrane and an Fe electrodeposited on stainless steel mesh (400 mesh) cathode. Hydrogen gas was introduced at the anode side of the gas flow cell and nitrogen gas was passed through the cathode side at a rate of 180 mL/min each. The pressure of gas inside the cell was 1 atmosphere. A constant potential of -1.2V relative to the anode was applied to the cathode for 30 mins.

[0126] The gases flowing through the cell were combined and bubbled through a 40 ml_ solution of 1 mM h^SCu and then through a 20ml_ solution of 1 mM H₂S0₄. The gas was bubbled throughout the period of applied potential and for 30 minutes after it was finished.

[0127] Ammonia produced over 30mins of applied potential was 87.0 nmoles with a Faradaic efficiency of 24.9%.

Example 11 : Preparation and Use of a Gel Membrane with [C₄mpyr][eFAP] liquid salt suitable for use as a free standing membrane

[0128] A membrane was prepared as for Example 10 except that the liquid salt to polymer ratio was 7:3 by weight and the solution was cast to form a free-standing membrane. Ammonia produced over 30mins of applied potential was 59.5 nmoles with a Faradaic efficiency of 71.5%.

Example 12: Preparation and Use of a Nation™ Gel Membrane with a [C₄mpyr][eFAP] liquid salt

[0129] A cell was constructed as described in Example 10, except that the electrolyte comprised a Nafion™-gel membrane and was prepared as described in the following example. [0130] A solution containing 1 to 9 ratio by weight of Nafion™ and [C₄mpyr][eFAP] was prepared by mixing 9 parts of [C₄mpyr][eFAP] and 20 parts of a 5 w/w% solution of Nafion in lower aliphatic alcohols and water, as supplied by Aldrich. This was dropped on to a 34mm diameter Solupour™ membrane to achieve complete coverage of the solution over the membrane. It was then dried overnight in an oven at 60°C. The gas flow cell was constructed with this Nafion™-gel membrane between the cathode and the anode. Ammonia produced was found to be 93.1 nmoles with a Faradaic efficiency of 30.6%.

Example 13: Preparation and Use of a free-standing electrolyte membrane comprising a Nafion™ Membrane with a liquid salt

[0131] A cell was constructed as described in Example 11 except that the membrane was a Nafion™-gel membrane. The membrane was prepared as follows. A 3 to 7 weight ratio mixture of Nafion™ and liquid salt was prepared as described in example 12 and drop cast. It was then dried overnight in an oven at 60°C.

[0132] It will be apparent to the person skilled in that further variations of the cell described above can be constructed, such as, by excluding Solupour™ as the support for the electrolyte membrane or using other supports.

Example 14: N₂ reduction in [P6,6,6,14][PFB]

[0133] Following the same methods as Example 3, [P₆,6,6,14][PFB] was used as the liquid salt electrolyte. A constant potential of 0.8 V vs NHE was applied for 3hrs. The Faradaic efficiency for ammonia production in this system was 18%.

Example 15: N₂ reduction on Ru catalyst.

[0134] Following the same procedure as Example 3, a ruthenium film was deposited onto the working electrode. A constant potential of -0.8 V vs NHE was applied on the working electrode for 30 min. The current density was approximately twice that of Example 3. Ammonia measurement shows that the Faradaic efficiency for ammonia production in this system was 28%. Use of other Electrode substrates

[0135] As the N₂ reduction currents obtained on the flat fluorine doped tin oxide glass substrate are too small to be practical, a variety of high surface area and porous substrates such as nickel foam, stainless steel mesh were used to increase the working electrochemical surface area. The current density increases on using the porous substrates, for example the current increases from 0.012 A m^"2 for fluorine doped tin oxide to 0.1 A.m^"2 for stainless steel mesh, 0.25 A.m^"2 for nickel foam, with corresponding Faradaic efficiency of 59% and 45% respectively. Due to the increased current, the production rate increases remarkably from 2.9 mg.m^"2 h^"1 on fluorine doped tin oxide substrate to 14 mg.m^"2 h^"1 on the stainless steel substrate. It is important to note that the stainless steel cathodes used here are quite thin and flexible so that there is considerable scope to further increase yields by optimizing cathode thickness and also to implement, high surface area, thin-layer or spiral wound type cell constructs.

[0136] Compared with [P6,6_,6,i4][eFAP], the [C₄mpyr][eFAP] liquid salt produces a higher current. This is probably due to its lower viscosity, which promotes better mass diffusion during the reaction. The Faradaic efficiency for N₂ reduction for the stainless steel electrode in [C₄mpyr][eFAP] is a little lower, as expected as the N₂ solubility in this ionic liquid at 0.20 mg/g is lower than [P6,6,6,i4][eFAP] at 0.28 mg/g. Nonetheless, the yield is higher in [C₄mpyr][eFAP].

[00100] The reason for the high solubility of N₂ in the selected ionic liquids is further examined by DFT calculations. The interaction of various common anions with N₂ is relatively weak with ions such as CI^" and with fluorinated anions such as BF₄ ^" and PF6^". On the other hand, the interaction with [eFAP] is distinctly stronger.

[00101] Without wishing to be bound by theory it may be that two modes are distinguished in the calculations; the first reveals the N₂ interacting with the F-atoms attached to the alkyl chains, while the second has the N₂ interacting with the F atoms bound to the phosphorous, the latter being the stronger interaction. It appears from all of these calculations that the more strongly delocalized the charge is onto the neighbouring groups, the stronger is the N₂ binding interaction. The distinction between the binding energy in this eFAP complex and that with PF6^" is striking; in the eFAP case the additional derealization of charge onto the three C2F5 groups modulates the charge on the P-bound fluorines and thereby enhances the interaction with N₂. Introducing the cation into these calculations shows that the interaction becomes even stronger because of further charge interaction between the anion and the cation.

COMPARATIVE EXAMPLES

[0137] While the prior art includes general disclosure of the use of electrolytes comprising liquid salts, many of these electrolytes are unsuitable and/or have an untenably low Faradaic efficiency for reduction of dinitrogen according to the method of the present invention.

[0138] This can be illustrated, for example with reference to U.S. Patent 2016/0138176 (Yoo et al) which describes a method of synthesizing ammonia using an electrolysis cell containing an aqueous, or liquid electrolyte of certain alkali metal (salts) or an ionic liquid. In particular, Comparative Examples 16 and 17 relate to salts disclosed in US 2016/0138176 however in the method of the present invention, the salt of Comparative Example 16 results in an untenably low Faradaic efficiency while the salt of Comparative Example 17 becomes unstable.

[0139] Comparative Example 18 illustrates more generally that certain ionic salts become unstable when used in the method of the present invention and for that reason are unsuitable.

Comparative Example 16: Reduction of N2 in [HMIM][NTf2]

[0140] N₂ reduction was carried out as described in Example 2 except that the liquid salt comprised the electrolyte [HMIM][NTf₂] and the working electrode employed the catalyst of Example 1. Since ionic liquids are generally regarded as salts having a melting point < 100°C [HMIM][NTf₂] may be regarded as an ionic liquid. As the melting point of this salt is around 70°C the reduction was carried out at 70°C in the liquid state.

[0141] The Faradaic efficiency in this case was only 0.64%. Comparative Example 17: Reduction of N₂ in [EMIM][B(CN)₄]

[0142] N₂ reduction was carried out as described in Example 2 except that the electrolyte comprised the liquid salt [EMIM][B(CN)₄] and the working electrode employed the catalyst of Example 1 .

[0143] The apparent Faradaic efficiency in this case was only 14%. However the same apparent Faradaic efficiency was measured when an argon gas feed to the cell was used, replacing the N₂ feed. Since in the latter case the ammonia is not produced from the supplied N₂ it is being formed by decomposition of the electrolyte comprising the liquid salt, indicating that such cyano-group containing liquid salts when used as electrolytes are not stable against reduction under these conditions.

Comparative Example 18 : Reduction of N₂ in [EMIM][eFAP]

[0144] N₂ reduction was carried out as described in Example 2 except that the electrolyte comprised the liquid salt [EMIM][eFAP] and the working electrode employed the catalyst of Example 1 .

[0145] Ammonia production measurements show that both argon and N₂ saturated electrolytes produce ammonia (or a detectable amine), and the Faradaic efficiency% is 35% and 16% respectively. This indicates that the ionic liquid decomposes during the electrolysis to produce amine products and ammonia. This liquid salt is therefore not suitable for nitrogen reduction to ammonia.

[0146] While this invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification(s). This application is intended to cover any variations uses or adaptations of the invention following in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth.

[0147] As the present invention may be embodied in several forms without departing from the spirit of the essential characteristics of the invention, it should be understood that the above described embodiments are not to limit the present invention unless otherwise specified, but rather should be construed broadly within the spirit and scope of the invention as defined in the appended claims. The described embodiments are to be considered in all respects as illustrative only and not restrictive.

[0148] Various modifications and equivalent arrangements are intended to be included within the spirit and scope of the invention and appended claims. Therefore, the specific embodiments are to be understood to be illustrative of the many ways in which the principles of the present invention may be practiced. In the following claims, means-plus-function clauses are intended to cover structures as performing the defined function and not only structural equivalents, but also equivalent structures.

[0149] "Comprises/comprising" and "includes/including" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. Thus, unless the context clearly requires otherwise, throughout the description and the claims, the words 'comprise', 'comprising', 'includes', 'including' and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

Claims (50)

1. A method for the electrochemical reduction of dinitrogen to ammonia, the method comprising the steps of:

(1 ) contacting a cathodic working electrode comprising a nanostructured catalyst and a counter electrode with an electrolyte comprising one or more liquid salts, wherein the liquid salt is formed by a combination of :

(i) a cation selected from the group consisting of PRi^ , NRi_₄ , C₄H8NR₂ wherein each R group is independently linear, branched or cyclic and preferably comprises from 1 to 18 carbon atoms, optionally partially or completely halogenated, optionally including a heteroatom, optionally including a functional group preferably chosen from ethers, alcohols, carbonyls, thiols, sulphoxides, sulphonates, amines, azos or nitriles, and wherein two R groups may connect to form a monocyclic or heterocyclic ring; and

(ii) an anion selected from the group consisting of (RO)_xPF6-_x , (RO)_xBF₄__x , R'S0₂NS0₂R', R'S0₂C(S0₂R')(S0₂R'), FS0₂NS0₂F, C₂0₄BF₂, C₂0₄PF_4j RC₂0₄BF₂, RC₂0₄PF₄, CF3SO3, R'S0₃, R'C0₂, CF3COO, R'_xPF₆-x, R'_XBF_4-X wherein each R' group is independently linear, branched or cyclic and preferably comprises from 1 to 18 carbon atoms, optionally partially or completely fluorinated and optionally including a functional group, preferably chosen from ethers, alcohols, carbonyls, thiols, sulphoxides, sulphonates, amines, azos or nitriles and wherein two R' groups may connect to form a monocyclic or heterocyclic ring..

(2) introducing dinitrogen and a source of hydrogen to the electrolyte, wherein the dinitrogen is reduced to ammonia at the cathodic working electrode.

2. A method according to claim 1 wherein the liquid salt is formed by a combination of: (i) a cation selected from the group consisting of C₄mpr, Ρε,θ,ε,Η, P(C2 fn)4 where Rf is a perfluoralkyl moiety; and

(ii) an anion selected from the group consisting of eFAP, NfO, PFO, FSI, NTf₂, B(otfe)₄ and CF₃COO.

3. A method according to claim 1 wherein the liquid salt is [P6,6,6, i4][eFAP].

4. A method according to claim 1 wherein the liquid salt is [C₄mpyr][eFAP].

5. A method according to claim 1 wherein the liquid salt is [P6,6,6, 1 ][FgC₄S03].

6. A method according to claim 1 wherein the liquid salt is [P6,6_,6, 14 ][PFO].

7. A method according to claim 1 wherein the liquid salt is [C₄mpyr][perfluorobutanesulfonate].

8. A method according to claim 1 wherein the liquid salt is [C₄mpyr][PFO].

9. A method according to claim 1 including the further step of raising the temperature of the electrolyte to between -35°C and 200°C, preferably 0°C to 150°C, more preferably 15°C to 130°C.

10. A method according to claim 1 including the further step of subjecting the electrolyte to a pressure between 0.7 bar nitrogen to 100 bar nitrogen, preferably 1 bar to 30 bar and most preferably 1 bar to 12 bar.

1 1 . A method according to claim 10 wherein the pressure is pulsed.

12. A method according to claim 1 wherein a current passing between the cathodic working electrode and the counter electrode is intermittent.

13. A method according to claim 1 further including the step of applying energy of ultrasonic frequency to the electrolyte.

14. A method according to claim 1 including the further step of humidifying dinitrogen gas, and passing the humidified dinitrogen gas in a stream over the cathodic working electrode.

15. A method according to claim 1 wherein the electrolyte further comprises a membrane.

16. A method according to claim 15 wherein the membrane is chosen from a polymer electrolyte, gelled ionic liquid electrolyte or porous separator.

17. A cell for electrochemical reduction of dinitrogen to ammonia, the cell comprising: a cathodic working electrode comprising a nanostructured catalyst for reduction of dinitrogen, a counter electrode, and an electrolyte comprising one or more liquid salts in contact with the working electrode wherein the liquid salt is formed by a combination of :

(i) a cation selected from the group consisting of PRi_-4 , NR- , C₄HSN R2 wherein each R group is independently linear, branched or cyclic and preferably comprises from 1 to 18 carbon atoms, optionally partially or completely halogenated, optionally including a heteroatom, optionally including a functional group preferably chosen from ethers, alcohols, carbonyls, thiols, sulphoxides, sulphonates, amines, azos or nitriles, and wherein two R groups may connect to form a monocyclic or heterocyclic ring; and

(ii) an anion selected from the group consisting of (RO)_xPF6-_x, (RO)_xBF₄__x, R'S0₂NS0₂R', R'S0₂C(S0₂R')(S0₂R'), FS0₂NS0₂F, C₂0₄BF₂, C₂0₄PF₄, RC₂0₄BF₂, RC₂0₄PF_4> CF₃SO₃, R'S0₃, R'C0₂, CF₃COO, R'_XPF_6-X (FAP), R'_XBF₄__X wherein each R' group is independently linear, branched or cyclic and preferably comprises from 1 to 18 carbon atoms, optionally partially or completely fluorinated and optionally including a functional group, preferably chosen from ethers, alcohols, carbonyls, thiols, sulphoxides, sulphonates, amines, azos or nitriles and wherein two R' groups may connect to form a monocyclic or heterocyclic ring.

18. A cell according to claim 17 wherein the liquid salt is formed by a combination of:

(i) a cation selected from the group consisting of C4impyr, Ρε,θ,θ,π, P(C2Rf_n)4 where R is a perfluoralkyl moiety); and

(ii) an anion selected from the group consisting of eFAP, NfO, PFO, FSI, NTf₂, B(otfe)₄ and CF₃COO.

19. A cell according to claim 17 wherein the liquid salt is [P6,6,6,i4][eFAP].

20. A cell according to claim 17 wherein the liquid salt is [C₄mpyr][eFAP].

21 . A cell according to claim 17 wherein the liquid salt is [P6,6,6,14 ][F9C₄S0₃].

22. A cell according to claim 17 wherein the liquid salt is [P6,6,6,14 ][PFO].

23. A cell according to claim 17 wherein the liquid salt is [C₄mpyr][perfluorobutanesulfonate].

24. A cell according to claim 17 wherein the liquid salt is [C₄mpyr][PFO].

25. A cell assembly formed by stacking in series two or more cells according to claim 17 or claim 18.

26. A cell according to claim 17 including heating means to maintain the temperature of the electrolyte between -35°C and 200°C, preferably 0°C to 150°C, more preferably 15°C to 130°C.

27. A cell according to claim 17 including pressurising means to subject the electrolyte to a pressure between 0.7 bar nitrogen to 100 bar nitrogen, preferably 1 bar to 30 bar and most preferably 1 bar to 12 bar.

28. A cell according to claim 17 adapted to pass an intermittent current between the cathodic working electrode and the counter electrode.

29. A cell according to claim 17 which further includes a humidifier for humidifying the dinitrogen gas, and adapted to pass the humidified dinitrogen gas in a stream over the cathodic working electrode.

30. A cell according to claim 17 wherein the electrolyte comprises a membrane.

31. A cell according to claim 17 wherein the membrane is chosen from a polymer electrolyte, gelled ionic liquid electrolyte or porous separator.

32. A cell according to claim 17 including an ultrasonic source for applying energy of ultrasonic frequency to the electrolyte.

33. A cell according to claim 17 wherein the reduction of dinitrogen to ammonia occurs principally in a region adjacent a three phase boundary on the working surface of the cathodic working electrode.

34. A cell according to claim 17 wherein the nanostructured electrocatalyst is adjacent an outer surface of the cathodic working electrode and creates a gas/electrolyte/metal three phase boundary region where electrolysis principally takes place.

35. A nanostructured catalyst for electrochemical reduction of dinitrogen to ammonia, the nanostructured catalyst comprising nanoparticles and having a high electrochemical working surface area as indicated by a double layer capacity, measured in an adjacent electrolyte layer, of greater than 0.1 mF/cm² and preferably greater 1 mF/cm² .

36. A nanostructured catalyst according to claim 35 comprising nanoparticles of one or more metals chosen from transition metals, or lanthanide metals, including inorganic compounds thereof.

37. A nanostructured catalyst according to claim 36 wherein the nanoparticles of one or more metals are supported on an electrically conductive, chemically inert support.

38. An electrolyte for electrochemical reduction of dinitrogen to ammonia according to the method of claim 1 , the electrolyte comprising one or more salts having dinitrogen solubility of at least 100 mg/L at the operating temperature and pressure of the cell, more preferably more than 200 mg/L and; most preferably greater than 400 mg/L and water solubility of less than 5 wt% at the operating temperature and pressure of the cell, more preferably less than 2% and most preferably less than 1 %.

39. An electrolyte according to claim 38 wherein the electrolyte further comprises a membrane.

40. An electrolyte according to claim 39 wherein the membrane is chosen from a polymer electrolyte or polymer electrolyte-liquid salt blend, or a gelled liquid salt electrolyte, or an electrolyte soaked into a porous separator .

41. An electrolyte according to claim 38 which further comprises a molecular liquid present in the liquid salt medium at a level between 90 vol% and 0.1 vol%, more preferably between 20 vol% and 0.2 vol% and most preferably between 50 vol% and 0.5 vol%.

42. An electrolyte according to claim 38 which further comprise a molecular liquid chosen from dimethylsulfoxide, tetraglyme and other oligio- and poly-ethers, glutaronitrile and other high boiling point nitriles, trifluorotoluene present in the liquid salt medium at a level between 90 vol% and 0.1 vol%, more preferably between 20 vol% and 0.2 vol% and most preferably between 50 vol% and 0.5 vol%.

43. An electrode for electrochemical reduction of dinitrogen to ammonia according to the method of claim 1 , the electrode having a hybrid structure comprising a porous conductive electrode material and nanostructured catalyst, wherein the electrode has an electrochemically active surface area and a multiplicity of pores adapted to allow both gas access and electrolyte access within the body of the electrode.

44. An electrode according to the claim 43 wherein the pores are at least partially filled with ion conducting material.

45. An electrode for electrochemical reduction of dinitrogen to ammonia according to the method of claim 1 , wherein the electrode is a membrane comprising porous metal or carbon impregnated with a catalyst material or a gelled liquid salt.

46. An electrolyte membrane comprising a thin layer of material in combination with one or more liquid salts for use in the cell of claim 1 , wherein the liquid salt is formed by a combination of:

(i) a cation selected from the group consisting of PRi_-4, NRi_-4, C₄H8NR2 wherein each R group is independently linear, branched or cyclic and preferably comprises from 1 to 18 carbon atoms, optionally partially or completely halogenated, optionally including a heteroatom, optionally including a functional group preferably chosen from ethers, alcohols, carbonyls, thiols, sulphoxides, sulphonates, amines, azos or nitriles, and wherein two R groups may connect to form a monocyclic or heterocyclic ring; and

(ii) an anion selected from the group consisting of (RO)_xPF6_-x, (RO)_xBF_4-x, R'S0₂NS0₂R', R'S02C(S02R')(S0₂R'), FS0₂NS0₂F, C₂0₄BF₂, C₂0₄PF₄, RC₂0₄BF₂, RC₂0₄PF₄, CF3SO3, R'S0₃, R'C0₂, CF₃COO, R'_XPF_6-X, R'_XBF_4- x wherein each R' group is independently linear, branched or cyclic and preferably comprises from 1 to 18 carbon atoms, optionally partially or completely fluorinated and optionally including a functional group, preferably chosen from ethers, alcohols, carbonyls, thiols, sulphoxides, sulphonates, amines, azos or nitriles and wherein two R' groups may connect to form a monocyclic or heterocyclic ring.

47 An electrolyte membrane according to claim 46 wherein the thin layer of material is chosen from the group comprising a polymer, a gel or a porous separator material.

48. An electrolyte membrane according to claim 46 wherein the liquid salt is in combination with a polymer.

49. An electrolyte membrane according to claim 46 wherein the liquid salt is in combination with a gell.

50. An electrolyte membrane according to claim 46 wherein the liquid salt is in combination with a porous separator.

51 A method for the electrochemical reduction of dinitrogen to ammonia according to claim 1 when carried out using the electrochemical cell of claim 17, the nanostructured catalyst of claim 35 and the electrolyte of claim 38.